In conventional LTE (Long Term Evolution) systems, when a UE (user equipment) keeps in connection with a cellular network, the UE can at the same time send or receive D2D (Device-to-Device) signals or channels, or send or receive D2D signals or channels in different serving cells.
D2D communications in LTE coverage occupy uplink resources, i.e., occupying uplink carriers of FDD (frequency division duplexing) cells or uplink sub-frames of TDD (time division duplexing) cells. A UE may send a cellular network communication channel or signal in one or multiple serving cells within an uplink sub-frame and at the same time, send or receive D2D signals or channels in another one or multiple serving cells within the same uplink sub-frame, as shown in FIG. 1. FIG. 1 illustrates a situation where sub-frames of multiple serving cells align with each other. A problem, which also exists in situations where sub-frames of multiple serving cells do not align with each other, lies in that: in an overlapping area of different uplink sub-frames (the overlapping area may be the overlapping area of uplink sub-frame n and uplink sub-frame n+1, or the overlapping area of uplink sub-frame n and uplink sub-frame n−1), a UE may send a cellular network communication channel or signal in one or multiple serving cells within an uplink sub-frame and at the same time, send or receive D2D signals or channels in another one or multiple serving cells within the same uplink sub-frame. Channels or signals of cellular networks include: PUSCH (physical uplink shared channel), PUCCH (physical uplink control channel), PRACH (physical random access channel), and SRS (sounding reference signal). Signals or channels of D2D services include SS (synchronization signal), DS (discovery signal), CCH (control channel) and DCH (data channel), and the like. Among the above signals or channels, D2D SS may include one or multiple of PD2DSS (primary D2D SS), SD2DSS (secondary D2D SS), and PD2DSCH (physical D2D synchronization channel). D2D DS may include D2D discovery signal sequences and D2D discovery signal messages.
D2D UEs may have different service requirements. For example, service requirements may include PS (public safety) requirements or N-PS (non-public safety) requirements. Different service requirements have different levels of importance.
D2D communications are divided into broadcast, groupcast and unicast according to different propagation manners. Broadcast refers to a manner where one UE sends information and all other UEs can receive the information. Groupcast refers to a manner where one UE sends information and all UEs in a group can receive the information. Unicast refers to a manner where one UE sends information and another UE receives the information.